1. Field of Invention
The present invention relates to a field of inspection on explosives, drug and the like, and in particular to an ion gate used in a dual ion mobility spectrometer (IMS) and the method thereof.
2. Description of Prior Art
Generally, a dual IMS is primarily formed of an ion source, two drift tubes (TOF), a positive and negative ion reaction zone, a positive ion gate, a negative ion gate and two detectors. The simplest formation is such that the two drift tubes are located on the two sides of the reaction zone, respectively. The dual IMS differs from a common IMS in that the structure of the ion gate imposes a significant effect on the sensitivity of the instrument due to the necessity of positive and negative ion detection. As shown in Patent Document 1 (U.S. Pat. No. 4,445,038), two electrodes arranged in the front of the drift tubes for positive and negative ions, respectively, forms the gates for positive and negative ions, and the ion source is located in the middle of the two electrodes. Sample gas is ionized after entering from a tube above the ion source, and stays within the gates for positive and negative ions at both sides of the ion source. After a pulse arrives, the positive and negative ions within the ion gates are released to the adjacent drift tubes, respectively. Patent Document 1 offers an advantage of a simple control of ions, while it has a disadvantage of complex manufacture process for the ion gate, strict requirements on assembly and high produce cost. Further, the effective utility rate of ions is low. The structure of the gates causes a loss of about 90% of the total ions inside the gates, leading to poor instrument sensitivity.
In order to improve the effective utility rate of ions, Patent Document 2 (U.S. Pat. No. 7,259,369 B2) provides a method of simultaneously storing positive and negative ions by use of a quad-polar ion trap and simultaneously releasing positive and negative ions under the control of electrodes. The quad-polar ion trap is composed of two oblate cylinders, an external cylinder with a larger inner radius and two smaller hat-shaped cylinders each having a hole in the center. The two oblate cylinders are assembled at both ends of the external cylinder, and the two smaller hat-shaped cylinders are assembled inside the two oblate cylinders, respectively, with their hat tops opposite to each other. The structure in Patent Document 2 eliminates the disadvantage in Patent Document 1, because the quad-polar ion trap has a function of focusing and compressing ions and thus improves the system resolution, while there are several gas entrance holes allowing change of carrier gas and migrant gas at any time. Unfortunately, both of the positive and negative ions are stored in the same area in the ion trap, and thus part of the ions is lost due to the charge exchange between the ions. Further, the quad-polar ion trap has a complex structure and thus a very stringent requirement for concentricity and assembly, leading to a higher fabrication cost. Also, the scheme of electrode control is relatively complicated, which makes control over the whole apparatus more difficult.
Another patent document provides a method of measuring positive and negative ions separately within a single drift tube under electrode control. This solution is advantageous in terms of a simple structure and small size of the apparatus, while the shortcoming is that it is impossible to measure both the positive and negative ions at the same time, and the change of carrier gas and migrant gas in the apparatus is restricted.